They'll set a range of prices - likely a lower, middle and upper price - for each model. Compensation rates for surrendered guns will be based on the age and condition of the firearm.

The information comes as the legislation banning military-style semi-automatics and assault rifles passed its second reading in Parliament.

Details won't be worked out by the time the bill passes, but Newshub understands the Government will release some detail on Wednesday about what advice they've been given by officials who ran Australian buybacks and the approach Police plans to take when considering what to pay for.

When asked if it's wise to pass legislation without having a process of how that buyback is going to work, Police Minister Stuart Nash said: "What I will say is police are already collecting bank account numbers."

When it comes to the cost, Newshub's been told the Government is hoping to have details of that out by next week.

National had raised concerns in the Select Committee that "vital details of the amnesty and firearm buy-back scheme have not been disclosed".

It's yet to be decided if surrendered firearms will be destroyed, nor how that will be done and who will be responsible.

By Thursday, the ban will become law - but there are still big questions about how the amnesty will work.

"We're working through that at the moment and we're coming up with what we believe will be a good scheme," Nash said.

National's Judith Collins says the whole ban's success rides on the success of the buyback.

"That is crucial, otherwise why bother?"

But that success may be difficult to measure, because the Government doesn't know how many guns there are in New Zealand, and it won't know how many remain in the community once the amnesty is finished.

"We will not have any idea, but if people are found in possession of an illegal weapon they'll face five years in prison," Nash said.

The fate of the guns hasn't been decided yet either. The Police Minister said the New Zealand Defence Force (NZDF) has been charged with storing and transporting surrendered firearms, but Defence Minister Ron Mark hadn't yet been briefed

"When it comes down to the actual destruction of the firearms, I'll take advice as to whether we have the capacity, or whether or not police would intend doing it through another agency."

The NZDF itself is still awaiting details.

"NZDF will assist with planning and will likely provide support to any such operation; however the nature of any support is yet to be determined. Further questions will need to be directed to the NZ Police," a spokesperson said.

In the meantime, neither the Police Minister nor Police would tell Newshub how guns that have already been surrendered are being stored.

"I'm not 100 percent sure, but what I do know is they are stored in a way that is a lot safer than they were before they were handed in," Nash said.

Police sent Newshub a statement saying: "Police are considering a number of options for the collection and storage of firearms that the public will hand in to Police during the amnesty, but no decisions have been made yet."